The present invention relates to digital signal detecting circuitry and more particularly to a digital circuit which enables pulse width discrimination.
In a variety of electronic systems, information transmission and reception is often accomplished and controlled by the generation of pulses representing digital information. The pulses may be of a variety of configurations and may include modulation of their width, height and repetition frequency in order to convey certain information. Naturally, in any such system, the accuracy of the information transmitted is dependent upon the effective detection of the pulses so that unwanted or spurious signals may be separated from the transmitted pulses.
In one technique of transmitting information by digital pulses, the width of the transmitted pulses is fixed within predetermined limits so that the pulse may be distinguished from other signals. In this instance, if an unwanted signal of any magnitude, repetition or duration is received from a spurious source, it may be easily distinguished if its duration falls outside of the predetermined limits. By way of example, pulse width detection is used in aircraft transponders wherein the timing of a series of transmitted pulses is intended to elicit particular information from an aircraft. As can be appreciated, when such timing is critical to determining the mode of aircraft response, it is important that unwanted pulses are not detected as interrogation pulses so that they interfere with the interrogation timing sequence.
In the prior art, pulse width discrimination in transponder and other aircraft system has been generally accomplished by employing lumped delay lines and analog delays. Lumped delay lines, however, are bulky and cannot be easily used with the miniaturized digital circuits employed in new technology aircraft. Analog delay circuits are also bulky and suffer from additional deficiencies. Specifically, analog delays have a tendency to drift and require frequent adjustment in order to maintain the precise delay times needed for enabling pulse width discrimination. Neither of these techniques, are capable of providing the fixed, stable and reproducible delay times needed for distinguishing critical pulse widths. There is therefore a continuing need for digital circuitry which can provide such reproducible characteristics and enable pulse width detection and discrimination in a variety of digital circuits.
Accordingly, the present invention has been developed to overcome the shortcomings of the above known and similar techniques, and more particularly, to provide a digital pulse width discriminator for accurately detecting pulse width within precisely defined limits.